If you had told Calvin Dark three years ago that he’d be teaching Spin classes today, he never would have believed you.

When 32-year-old Calvin Dark bumps into people he hasn’t seen in a few years, they always ask, “What happened?” Part of it’s obvious. He shed 130 pounds from his formerly 345-pound frame. But what really happened was that he asked for help.

For years, he’d regularly show up at Balance Gym (and before that, Thomas Circle Sports Club, which was at the same location), get on the last treadmill in the corner to walk and then do some stationary biking. He never seemed to get any smaller, which was nothing new for Dark, who’d struggled with his weight his entire life.

But with the big 3-0 approaching and diabetes on both sides of his family, Dark got increasingly worried about what the extra pounds would mean for his long-term health. So when Balance’s Jermaine Ennis approached him to see whether he’d be interested in personal training, Dark went for it. “I had just paid my last car payment. I figured if I can give Hyundai hundreds of dollars a month, I could give it to myself for one month,” he says.

After Dark’s first session with Ennis, he felt — really felt — how much harder he had worked than usual. Despite the soreness, however, he appreciated that Ennis managed to hit the right level of intensity. It was enough to get results, but not so much that he felt frustrated and wanted to quit.

“Some people have the wrong impression of what personal training is for,” Dark says. “It’s not that I didn’t have the motivation.” What he needed was guidance.

Ennis persuaded him to talk to a nutritionist, which is another resource Dark had never considered. Although he wasn’t a junk food eater, the professional help steered him to some simple swaps. In his coffee and tea, he’d gotten into the habit of dumping in a ton of sugar. “By switching to Splenda, I saved hundreds of calories a day,” he says. He cut down on carbs and upped his daily allotment of veggies.

“It wasn’t a drastic diet, and I wasn’t working out eight times a week,” says Dark, who began with one personal training session each week before upping it to two. In 19 months, he reached the weight he’s now successfully maintained for two years. Looking back, Dark recognizes that he probably could have taken off pounds even faster if he’d chosen a more drastic approach. “But I like how I did it,” he says.

And Dark likes exercise a lot more now that he’s expanded his horizons beyond the treadmill. “I was the least coordinated person,” he says. He quickly proved that to Ennis when they were doing a boxing drill and Dark accidentally punched his trainer in the face. But gradually, Dark learned to feel comfortable in his body and around the gym.

Back before he’d started the personal training, Dark had attempted a Spin class. “I lasted 15 minutes; that was it,” he says. So when he got back on a bike two years later, he was surprised to find he didn’t just survive it. He loved it. After quickly becoming a regular at Stacy Martin’s classes, she persuaded him to get certified to teach.

Now instead of getting personal training, Dark is leading classes. His message to students? “If I can do it, you can do it.” And he’s there to help.